This invention generally relates to sensors and methods for detecting analytes. More particularly, this invention relates to optical sensors, sensor arrays, sensing systems and sensing methods for intelligent sensing and detection of unknown materials by way of real-time feedback and control of sampling conditions.
U.S. Pat. No. 4,859,864 to Smith discloses an air bubble sensor that employs light emitting diode (LED) light sources, phototransistor detectors, and displays or alarms for detecting the presence of bubbles in a fluid sample.
U.S. Pat. No. 5,674,751 to Jaduszliwer, et al. disclose a hydrazine fuel fiber optic sensor that employs a diode laser pulsed light source, a calorimetric fiber optic sensor system, and a photodetector to detect changes in spectral absorption due to ppb levels of hydrazine fuel.
U.S. Pat. No. 5,445,795 to Lancaster, et al. disclose a portable optical sensor for detecting volatile organic compounds (VOCs) in vapors and aqueous media. The disclosed device comprises a vapochromic sensor formed from transition metal complex salts, a sensor chamber, a vacuum pump for drawing samples into the chamber, a light source for illuminating the sensor, a light detector responsive to light reflected from the vapochromic sensor, and a detection means for determining a color change in the sensor due to the presence of VOCs. In one disclosed embodiment for fuel tank sensing, the sensor, an LED illuminating light source, and a photodiode detector with an optical band-pass filter are all housed within the sensor chamber and a photodiode feedback signal is provided to a control means for adjusting a fuel metering valve via signal processing electronic circuitry. Other embodiments employ a bi-color LED that can be modulated between two wavelengths and gated detection electronics in the detector is synchronized with LED driver current to monitor small changes in reflected signals at both wavelengths.
U.S. Pat. No. 5,116,759 to Klainer, et al. discloses a vapor or liquid chemical sensor where analytes pass into a sampling cell where they contact sensing solutions for detection. The disclosed device comprises a single illumination source, an optional semi-permeable analyte membrane, a chamber with one or more analyte-sensitive solutions contained in a reservoir cell, a sample signal detector for detecting optical changes in the cell due to the analyte, and an optional reference signal detector for background signal correction. Reagent and sampling pumps are also disclosed for continuously flushing the cell with analyte and solution reagent. The disclosed device employs diodes, lasers or lamps as an excitation source, optically responsive analyte sensing solutions, detectors, and conventional electronic circuitry that are known in the art. In a preferred embodiment, an LED is the preferred light source and a photodiode is the preferred detector. Other embodiments disclose a light source sensor, a source stabilizer, a detector stabilizer, and a temperature sensor and compensator circuitry for feedback, monitoring and stabilizing the light source and detector. Disclosed embodiments include an A/D interface, alarms, display, recorders or plotters for readout, a computer and software.
Persaud and Dodd (Nature v. 299, pp. 352-355, Sep. 23, 1982) disclose an electronic nose comprised of semi-selective sensors in a cross-reactive sensor array designed to mimic a mammalian olfactory system. The disclosed sensors comprise commercially available semiconductor transducer gas sensors that exhibit a conductance change when the adsorb ambient vapors. The disclosed sensors were capable of detecting vapors at high concentrations ranging from 0,1 to 10 mols per liter of air. The response time for these sensors ranged from 1 to 3 minutes. Measurements made with various sensor parings demonstrated selectivity toward a number of analyte vapors at high concentrations.
U.S. Pat. No. 5,512,490 to Walt and Kauer disclose a fiber optic sensor with semi-selective sensors in a cross-reactive sensor array that employs spectral recognition patterns for identifying and detecting a variety of analytes. The reference teaches thin film sensors formulated by mixing polymers with dye compounds. The sensors are immobilized on either a solid planar translucent or transparent substrate or a fiber optic fiber or bundle. In a preferred embodiment, the substrate is a transparent optical fiber bindle in which sensors are placed on the ends of optical fibers or groups of such fibers. The sensing system taught by this reference utilizes an arc lamp excitation source, an optical train comprising a series of lenses, filters which are sequentially switched to provide for changes in both excitation light wavelength and emitted light wavelength, and a CCD camera detector which captures spatial images of the fluorescence intensity of individual sensor elements at various wavelengths. The measured responses of individual sensors to analytes are combined to form a pattern of spectral responses over time that are unique to a specific analyte. Spectral response patterns are stored in a library and the response patterns generated from unknown samples are compared with library patterns to identify and detect target analytes. Either light intensity or wavelength may be employed for analyte determinations
U.S. Pat. No. 5,063,164 to Goldstein discloses a biomimetic sensor for detecting airborne toxins. The disclosed device comprises a porous, semi-transparent substrate which is sufficiently transmissive to light to permit detection of transmitted light by an LED and photodiode and is impregnated with a self-regenerating sensor. The sensor allegedly mimics the human response to toxins with regard to sensitivity and affinity by employing a molecular encapsulant that contains a chemical sensor reagent. The disclosed device provides for detecting a change in optical density of the sensor which is dependent on toxin concentration and time of exposure. For dilute analyte levels, extended exposure times are required for adequate sensitivity and detection.
Smardzewski [Talanta 35(2):95-101(1988)] discloses a multi-element optical waveguide sensor for detecting analytes in fluids which comprises eight fiber optic waveguides each circumferentially coated with sensing material, an array of eight sequentially-activated LEDs optically coupled to the waveguide assembly, and a single detector or array of multiple detectors, photomultiplier tubes or photodiodes, optically coupled to the waveguide assembly. Samples are passed over the outer surface of the coated waveguides and color changes produced by analyte interaction with the coating are monitored. In the disclosed method, each channel is sampled sequentially with measurements made on a single channel before moving to a subsequent channel. In the disclosed method the LEDs are pulsed on and off with switching times of at least one millisecond during measurements. The device provides for sensor signal output to be visually displayed or input to a microprocessor pattern-recognition algorithm. CMOS analog switches/multiplexers are used in feedback loops to control automatic gain-ranging, light-level adjustment and channel-sequencing. The detection limit and sensitivity of the disclosed device and method are limited to ppm levels.
Kopola, et al. [SPIE, Fiber Optic Sensors, v. 586, pp. 204-210 (1985)] disclose an eight channel spectrophotometer for measuring spectral reflectance at discrete wavelengths. The disclosed device comprises eight different LED light sources that cover a wavelength range between 480 nm and 1500 nm, a reference and measurement photodiode detector, a temperature controller, a fiber optic probe, signal conditioning electronics, microprocessor controller, and a display and plotter interface. In the disclosed method, measurements of both a reference LED output signal and sample LED output signal, which is modulated by the presence of an analyte, are simultaneously made with a single LED source and each reference and measurement detectors. With the disclosed device and method, sample measurements are time multiplexed with measurements made sequentially for each individual LED channel.
Hauser, et al. [Meas.Sci.Technol. 6:1082-1085(1995)] disclose a chemical sensor comprising LED light sources and filtered sample and reference photodiode detectors coupled to a fiber optic for detecting the optical response of a sensing membrane to analytes. The LED is modulated at 2 kHz. The disclosed device provides for a light demodulator for background signal corrections. Detector and reference signals are ratioed to compensate for instability in the LED light source.
The sensitivity of the disclosed device and method apparently is limited to 0.2% or 2000 ppm detection limits. Disclosed sampling times of several minutes or more are apparently required.
Bruno, et al. [Anal.Chem. 69(3):507-513(1997)] disclose a six channel sensor array for detecting blood analytes. The disclosed device comprises LED light sources, excitation and emission filters, photodiode detectors, pH membrane sensors and electronic circuitry. The device provides for modulating LED driving current and photodiode gain factors and providing output to a computer via an A/D/converter for display and analysis of data and control of fluid flow to the sensor. The disclosed sensor response time is approximately 30 seconds with a sampling time ranging from 1 to 15 minutes for each sensor. Sensitivity of the device is limited by signal noise caused by temperature and pressure variations due to sample fluid flowing through the sensor cell. An additional limitation with the disclosed device and method is a diminished responsivity of sensors with extended light exposure during sampling due to photobleaching.
Holobar, et al. [Anal.Methods and Instrum. 2(2):92-100(1995)] disclose a double-beam, flow-through pH sensor that employs a sample solution pump, an LED light source and two filtered photodiodes, one as a reference detector and the other as a sample detector. The disclosed sensor response time is approximately 20-30 seconds.
Boisde, et al. [Chemical and Biochemical Sensing with Optical Fibers and Waveguides, Artech House (Boston, 1996)] have reviewed the state of fiber optic chemical sensor art and have shown that LED excitation light sources, photodiode detectors, and multi-channel sensor wavelength multiplexing and spatial multiplexing are known in the art.
Taib, et al. [Analyst 120(6):1617-1625(1995)] have reviewed solid-state fiber optic sensor instrumentation and have shown that LED light sources, fiber optic light guides, optical transducers for analyte detection, amplifiers, signal processors and output devices are all known in the art of chemical sensor technology. The authors note that LEDs are particularly amenable to high frequency electronic modulation, that the response time of photodiode detectors was in the microsecond range, and that the use of multiple sensor channels with filtered LEDs and photodiodes and microprocessor control of pulsed of LED sources can provide advantageous simultaneous multi-channel/multi-parameter measurements. The authors additionally note that multi-channel sensors may be coupled to microprocessors to carry out parallel signal processing under software control and thereby exploit the capabilities of pattern recognition and artificial neural network methods.
Despite the many advantageous features provided by current chemical sensor technology, there is a need for a chemical sensor, sensing system and sensing method which provide for a multi-sensor, cross-reactive, sensor array having a rapid response time, a rapid sampling time, dynamic modulation of sampling and detection parameters, intelligent feedback control of analyte sampling conditions, smart mode sampling, smart detection through application of sophisticated analyte detection algorithms, and high sensitivity, discrimination, and detection capability for a variety of target analytes at sub ppm to ppb level concentrations.
The present invention relates to a chemical sensor, sensing system and sensing and identification method which provide for a multi-sensor, cross-reactive, sensor array having a rapid response time, a rapid sampling time, dynamic modulation of sampling and detection parameters, intelligent feedback control of analyte sampling conditions, smart mode sampling, smart detection through application of sophisticated analyte detection algorithms, and high sensitivity, discrimination, and detection capability for a variety of target analytes at sub ppm to ppb level concentrations.
One object of the present invention is to provide a relatively inexpensive, robust, dynamically configurable, portable sensing device.
An additional object of the present invention is to provide for porous or fibrous sensor substrates which enhance the responsivity, selectivity, and discrimination of sensors for target analytes.
A further object of the present invention is to provide for real-time, dynamic configuration of sensor excitation sources, detectors, sampling time and sampling rate to optimize sensor responsivity and selectivity for target analytes in a given sampling environment.
A yet further object of the present invention is to provide for rapid sensor response and rapid detection of low level signals for monitoring sensor temporal response profiles in detecting and discriminating target analytes.
A still further object of the present invention is to provide for an intelligent or xe2x80x9csmartxe2x80x9d nose that mimics the highly sensitive and discriminating vapor detection capability of olfactory systems of animals
An additional object of the present invention is to enable sampling under both negative and positive ambient pressure conditions.
A further object of the present invention is to provide for intelligent sensing of target analytes through electronic modulation of sampling conditions, such as flow rate, sampling duration, and sensor temporal response profiles by way of computer-controlled feedback.
An additional object of the present invention is to provide for removable, interchangeable sensor array substrates for rapidly changing sensor materials and sensor sites in the arrays for either targeting specific analytes or replacing spent sensors when they lose their responsivity to analytes due to either photo-bleaching or chemical reaction.
A further object of the present invention provides for utilization of a wide variety of sensor materials, such as dyes, dye-polymers, and polymers conjugated with dyes, which would normally be considered less suitable with conventional sensing devices due to relatively small analyte response signals.
An additional object of the present invention provides for multiple, cross-reactive sensors deployed in a sensor array for detecting and discriminating a wide variety of target analytes in complex sample mixtures.
Yet another object of the present invention is in providing directly illuminated sensors that do not require epi-illuminating optics which produce undesirable optical signal losses at low response levels.
A further object of the present invention is in providing real-time response signal baseline resetting and high gain response signal amplification tailored to individual sensor elements to avoid detector saturation, eliminate background fluorescence, and provide for simultaneous sampling and discrimination with all sensor elements in the array regardless of relative sensor responsivity to analytes.